For us personally basically only treadmills are of interest for our application, such as the Schiller CS 200:

But what about 1 cm GPS? Why does it not work for our application?

https://en.wikipedia.org/wiki/Real-time_kinematic_positioning

In practice, RTK systems use a single base-station receiver and a number of mobile units. The base station re-broadcasts. With smartwatches this seems not to be possible:

Accuracy of satellite positioning using GNSS receivers in sports watches

https://www.sciencedirect.com/science/article/pii/S0263224124003117